Dynamic control of cystic fibrosis transmembrane conductance regulator Cl(-)/HCO3(-) selectivity by external Cl(-)

• Published in: The Journal of biological chemistry Vol. 279; no. 21; p. 21857
• Main Authors: Shcheynikov, Nikolay, Kim, Kil Hwan, Kim, Kyung-Mi, Dorwart, Michael R, Ko, Shigeru B H, Goto, Hidemi, Naruse, Satoru, Thomas, Philip J, Muallem, Shmuel
• Format: Journal Article
• Language: English
• Published: United States 21-05-2004
• Subjects: Animals; Chloride-Bicarbonate Antiporters - biosynthesis; Chloride-Bicarbonate Antiporters - chemistry; Chlorine - metabolism; Cystic Fibrosis Transmembrane Conductance Regulator - chemistry; Dose-Response Relationship, Drug; Electrophysiology; Female; Hydrogen-Ion Concentration; Membrane Potentials; Mice; Oocytes - metabolism; Pancreas - metabolism; Patch-Clamp Techniques; Protein Conformation; RNA, Complementary - metabolism; Salivary Glands - metabolism; Time Factors; Trypsin - pharmacology; Xenopus; Xenopus laevis
• ISSN: 0021-9258
• DOI: 10.1074/jbc.M313323200

HCO(3)(-) secretion is a vital activity in cystic fibrosis transmembrane conductance regulator (CFTR)-expressing epithelia. However, the role of CFTR in this activity is not well understood. Simultaneous measurements of membrane potential and pH(i) and/or current in CFTRexpressing Xenopus oocytes revealed dynamic control of CFTR Cl(-)/HCO(3)(-) permeability ratio, which is regulated by external Cl(-) (Cl(-)(o)). Thus, reducing external Cl(-) from 110 to 0-10 mm resulted in the expected increase in membrane potential, but with no corresponding OH(-) or HCO(3)(-) influx. Approximately 3-4 min after reducing Cl(o)(-) to 0 mm, an abrupt switch in membrane potential occurs that coincided with an increased rates of OH(-) and HCO(3)(-) influx. The switch in membrane permeability to OH(-)/HCO(3)(-) can also be recorded as a leftward shift in the reversal potential. Furthermore, an increased rate of OH(-) influx in response to elevating pH(o) to 9.0 was observed only after the switch in membrane potential. The time to switch increased to 11 min at Cl(o)(-) of 5 mm. Conversely, re-addition of external Cl(-) after the switch in membrane potential did not stop HCO(3)(-) influx, which continued for about 3.9 min after Cl(-) addition. Importantly, addition of external Cl(-) to cells incubated in Cl(-)-free medium never resulted in HCO(3)(-) efflux. Voltage and current clamp experiments showed that the delayed HCO(3)(-) transport is electrogenic. These results indicate that CFTR exists in two conformations, a Cl(-) only and a Cl(-) and OH(-)/HCO(3)(-) permeable state. The switch between the states is controlled by external Cl(-). Accordingly, a different tryptic pattern of CFTR was found upon digestion in Cl(-)-containing and Cl(-)-free media. The physiological significance of these finding is discussed in the context of HCO(3)(-) secretion by tissues such as the pancreas and salivary glands.